Since its inception, membrane technology has played an important role towards improving the performance of a large number of industrial processes. Membranes may be considered one of the most versatile separation technologies available today as they may be successfully adapted for a wide range of applications involving solid/liquid, gas/gas, gas/liquid, and liquid/liquid separation processes. See for example, O. O Hart, R. C. Squires, The role of membrane technology in industrial water and wastewater treatment, Desalination, 56 (1985) 69-87; Y. Lee, S. Ahmed, Membrane technologies: Industry trends and applications, Membrane Technology, 98 (1998) 11-12; Ann-Sofi Jönsson, Gun Trägardh, Ultrafiltration applications, Desalination, 77 (1990) 135-179; and E. Drioli, E. Fontananova, Membrane technology and sustainable growth, Chemical Engineering Research and Design, 82(A12-2004) 1557-1562. One such application is water treatment. Due to population growth and urbanization, the resulting increase in global water scarcity and decrease in source water quality are expected to be followed by significant growth in the application of membrane separation processes for drinking water and wastewater treatment. According to a report prepared by the Freedonia Group (The Freedonia Group, Executive summary, World membrane separation technologies—Industry Study with Forecasts for 2012 & 2017. Study #2468, April 2009), the world membrane market in 2012 is predicted to grow to 15 billion dollars. More than 50% of this value will be related to the water and wastewater treatment markets associated with microfiltration, ultrafiltration, and reverse osmosis membranes. Similar figures were presented in a 2005 study by Leiknes that indicated the area with the greatest potential for market growth were microfiltration and ultrafiltration membranes for membrane bioreactors systems (TorOve Leiknes, Membrane technology in environmental engineering—meeting future demands and challenges of the water and sanitation sector, Desalination, 199 (2006) 12-14).
Even with the high potential for application of microfiltration and ultrafiltration membranes, a number of important scientific and engineering issues still need to be addressed such as the development of membranes with a reduced susceptibility towards fouling and understanding the fundamental processes that control membrane formation, morphology, and performance. Fouling is the complex interaction mechanism between the membrane and the diverse variety of species that are present in natural waters and wastewaters including ions, organics, and colloidal particles. See for example, G. Crozes, C. Anselme, G. Mallevialle, Effect of adsorption of organic matter on fouling of ultrafiltration membranes, Journal of Membrane Science, 84 (1993) 61-77; Ana Rita Costa, Maria Norberto de Pinho, Menachem Elimelech, Mechanisms of colloidal natural organic matter fouling in ultrafiltration, Journal of Membrane Science, 281 (2006) 716-725; and W Gao, Heng Liang, Jun Ma, Mei Han, Zhong-lin Chen, Zheng-shuang Han, Gui-bai Li, Membrane fouling control in ultrafiltration technology for drinking water production: A review, Desalination, 272 (2011) 1-8. Many studies have reported the viability of microfiltration and ultrafiltration for water and wastewater treatment; however, in most cases it was necessary to devise pretreatment and/or operational strategies to overcome the issues with fouling to obtain satisfactory results. See for example, J. C. Mierzwa, I. Hespanhol, M. C. C. da Silva, L. D. B. Rodrigues, C. F. Giorgi, Direct drinking water treatment by spiral-wound ultrafiltration membranes, Desalination, 230 (2008) 41-50; A. W. Zularisam, A. F. Ismail, M. R. Salim, Mimi Sakinah, T. Matsuura, Application of coagulation-ultrafiltration hybrid process for drinking water treatment: Optimization of operating conditions using experimental design, Separation and Purification Technology, 65 (2009) 193-210; A. N. Janssen, J. van Agtmaal, W. B. P. van den Broek, A. J. Geilvoet, H. W. H. Menkveld, J.-C. Schrotter, J. H. J. M. van der Graaf, Prefiltration of wastewater effluent: Effects on foulants and performance during dead-end ultrafiltration, Desalination, 250 (2010) 855-860; and Xing Zheng, Mathias Ernst, Martin Jekel, Stabilizing the performance of ultrafiltration in filtering tertiary effluent—Technical choices and economic comparisons, Journal of Membrane Sciences, 366 (2011) 82-91.
The challenge for synthesizing membranes with higher rejection capacity and higher flux has been pursued for many researchers since the beginning of membrane technology development (Mark C. Porter, Ultrafiltration, Chapter 3—Handbook of industrial membrane technology. Edited by Mark C. Porter, Noyes Publications, 1990). To accomplish this aim, the use of additives in the dope casting solution or in the coagulations bath is widely applied in the phase inversion casting process (Zhen-Liang Xu, F. AlsalhyQusay, Polyethersulfone (PES) hollow fiber ultrafiltration membrane prepared by PES/non-solvent/NMP solution, Journal of Membrane Science, 233 (2004) 101-111 and G. Arthanareeswaran, D. Mohan, M. Raajenthiren, Preparation, characterization and performance studies of ultrafiltration membranes with polymeric additive, Journal of Membrane Science, 250 (2010) 130-138). The use of polyvinylpyrrolidone, polyethylene glycol, and others organic additives is extensively reported to improve polymeric membranes permeability and fouling resistance (Jian-Jun Qin, Fook-Sin Wong, Ying Li, Yu-Tie Liu, A high flux ultrafiltration membrane spun from PSU/PVP(K90)/DMF/1,2-propanediol, Journal of Membrane Science, 211 (2003) 139-147; AniIdris, Norashikin Mat Zain, M. Y. Noordin, Synthesis, characterization and performance of asymmetric polyethersulfone (PES) ultrafiltration membranes with polyethylene glycol of different molecular weights as additives, Desalination, 207 (2007) 324-339; HeruSusanto, Mathias Ulbricht, Characteristics, performance and stability of polyethersulfone ultrafiltration membranes prepared by phase separation method using different macromolecular additives, Journal of Membrane Science, 327 (2009) 125-135; and Weifeng Zhao, Jingyum Huang, Baohang Fang, ShengqiangNie, Nan Yi, Baihai Su, Haifeng Li, Changsheng Zhao, Modification of polyethersulfone membrane by blending semi-interpenetrating network polymeric nanoparticles, Journal of Membrane Science, 369 (2011) 258-266). With the advances occurred in the production of nanomaterials, a new generation of nanoparticles-based membranes started to be developed all around the word, as it is described in the work developed by Kim and Van der Bruggen (The use of nanoparticles in polymeric and ceramic membrane structures: Review of manufacturing procedures and performance improvement for water treatment, Environmental Pollution, 158 (2010) 2335-2349). As it was conceived for most additives employed for membrane casting, the use of nanoparticles is intended for improving membrane performance, mainly flux enhancement, and fouling resistance (Law Yong Ng, Abdul Wahab Mohammad, ChoePeng Leo, NidalHilal, Polymeric membranes incorporated with metal/metal oxide nanoparticles: A comprehensive review, Desalination, xxx (2010) xxx-xxx. In press). Considering its outstanding intrinsic chemical, thermal and mechanical properties, polyethersulfone (PES) has been chosen for many researchers as the base polymer for nanocomposite ultrafiltration membranes preparation.
The use of TiO2 nanoparticles for casting PES composite membrane has been reported by Guiping Wu et al. (Preparation and characterization of PES/TiO2 composite membranes, Applied Surface Science, 254 (2008) 7080-7086), with the production of membranes with enhanced performance, such as permeability, and fouling resistance, but with no significative changes in the membrane structure. Similar results were obtained by Jing-Feng Li et al. (Effect of TiO2 nanoparticles on the surface morphology and performance of microporous PES membranes, Applied Surface Sciences, 255 (2009) 4725-4732), but some changes in membranes structure had been observed. In the work developed by Jiang-nan Chen et al. (Preparation and characterization of PES/SiO2 organic-inorganic composite ultrafiltration membrane for raw water pretreatment, Chemical Engineering Journal, 168 (2011) 1272-1278) it is reported that the use of SiO2 nanoparticles for casting PES composite membranes was effective for increasing membrane permeability and fouling resistance, but no obviously changes in membranes structure were observed. The use of Al2O3 nanoparticles for PES ultrafiltration membrane casting was evaluated by NermemMaximous et al. (Preparation, characterization and performance of Al2O3/PES membrane for wastewater filtration, Journal of Membrane Science, 34 (2009) 67-75), indicating that the use of these nanoparticles is effective for improving membrane permeability and fouling resistance, but there is no mention about the correlation with the membrane structure. In a different work, NermemMaximous et al. (Optimization of Al2O3/PES membranes for wastewater filtration, Separation and Purification Technology 73 (2010) 294-301) focused on the optimization of PES and Al2O3 nanoparticles composite membranes for wastewater treatment, obtaining similar results for membrane permeability and fouling resistance when Al2O3 nanoparticles were incorporated in the membrane. In an attempt to overcome the difficulties associated to the synthesis and application of nanocomposite membranes, StefenBalta et al. (The alternative of ZnO, Journal of Membrane Science, xxx (2011) xxx-xxx. In press), proposed the use of ZnO as an alternative to TiO2 nanoparticles for improving PES membrane performance. Results of this work showed that ZnO nanoparticles can improve membrane permeability and fouling resistance, but no definitive conclusions were drawn about the influence of membrane structure changes on these results. Many other works related to the use of nanomaterials for improving membranes performance using different polymers are also available elsewhere, all of them reporting some improvement in the membrane performance when variable amounts of specific nanoparticles are used in the casting process. See for example, Fu Liu, N. AwanisHashim, Yutie Liu, M. R. Moghareh Abed, K. Li, Progress in the production and modification of PVDF membranes, Journal of Membrane Science 375 (2011) 1-27; A. L. Ahmad, M. A. Majid, B. S. Ooi, Functionalized PSf/SiO2 nanocomposite membrane for oil-in-water emulsion separation, Desalination 268 (2011) 266-269; and Hosam A. Shawky, So-RyongChae, Shihong Lin, Mark R. Wiesner, Synthesis and characterization of a carbon nanotube/polymer nanocomposite membrane for water treatment, Desalination 272 (2011) 46-50.
The use of clay nanoparticles for the fabrication of composite membranes for water treatment is not extensively available in the literature. Clay nanoparticles have been successfully applied in the production of membranes for fuel cells (Dong Wook Kim, Hwa-Sup Choi, Changjin Lee, A. Blumstein, Yongku Kang, Investigation of methanol permeability of Nafion modified by self-assembled clay-nanocomposite multilayers, Electrochimica Acta 50 (2004) 659-662 and Rafael Herrera Alonso, Luis Estevez, HuiqinLian, AntoniosKelarakis, Emmanuel P. Giannelis, Nafion-clay nanocomposite membranes: Morphology and properties, Polymer 50 (2009) 2402-2410), and for gas separation (Guillaume Defontaine, Anne Barichard, SadokLetaief, ChaoyangFeng, Takeshi Matsuura, Christian Detellier, Nanoporous polymer—Clay hybrid membranes for gas separation, Journal of Colloid and Interface Science 343 (2010) 622-627). One of the first works on using modified and unmodified clay for Polysulfone (PSI) ultrafiltration composite membrane casting was developed by OriettaMonticelli et al. (OriettaMonticelli, Aldo Bottino, Ivan Scandale, Gustavo Capanelli, Saverio Russo, Preparation and properties of polysulfone-clay composite membranes, Journal of Applied Polymer Science 103 (2007) 3637-3644). In their work, membranes with increased permeability were obtained, with the higher increases for the membranes casted with modified clays. However, no specific correlation between membrane structure and permeability changes was presented, in comparison to the neat membrane. Another work where modified clay has been used as an additive for PSf composite membrane casting was developed by PriscilaAnadao et al. (Montmorillonite as a component of polysulfone nanocomposite membranes, Applied Clay Science 48 (2010) 127-132), but the focus of their work was related only to the changes in membrane morphology, thermal, mechanical and hydrophilic properties. No performance evaluations tests were carried out. The most recent work about clay and PES nanocomposite membranes was developed by NeginGhaemi et al. (Preparation, characterization and performance of polyethersulfone/organically modified montmorillonite nanocomposite membranes in removal of pesticides, Journal of Membrane Science 382 (2011) 135-147). In this work it is reported that increasing of organically modified clay concentration, up to 4%, results in significative changes in the membrane skin layer, and sub-layer, increasing the membrane permeability. It is also reported an increase in the pesticide retention capacity, with no significative changes in pore size.
Thus, the development of membranes with a lower susceptibility for fouling is of great need. There is also the challenge of rationally synthesizing membranes with increased flux and rejection capacity—a goal of many researchers since the beginning of membrane technology development (Mark C. Porter, Ultrafiltration, Chapter 3—Handbook of industrial membrane technology. Edited by Mark C. Porter, Noyes Publications, 1990). To accomplish this goal when forming membranes via phase inversion, one strategy is to use additives in the casting solution or in the coagulation bath (Zhen-Liang Xu, F. Alsalhy Qusay, Polyethersulfone (PES) hollow fiber ultrafiltration membrane prepared by PES/non-solvent/NMP solution, Journal of Membrane Science, 233 (2004) 101-111 and G. Arthanareeswaran, D. Mohan, M. Raajenthiren, Preparation, characterization and performance studies of ultrafiltration membranes with polymeric additive, Journal of Membrane Science, 250 (2010) 130-138). For example, the addition of high molecular weight polyvinylpyrrolidone, polyethylene glycol, or other organic additives has been reported to improve membrane permeability and fouling resistance. See for example, (Jian-Jun Qin, Fook-Sin Wong, Ying Li, Yu-Tie Liu, A high flux ultrafiltration membrane spun from PSU/PVP (K90)/DMF/1,2-propanediol, Journal of Membrane Science, 211 (2003) 139-147; Ani Idris, Norashikin Mat Zain, M. Y. Noordin, Synthesis, characterization and performance of asymmetric polyethersulfone (PES) ultrafiltration membranes with polyethylene glycol of different molecular weights as additives, Desalination, 207 (2007) 324-339; Heru Susanto, Mathias Ulbricht, Characteristics, performance and stability of polyethersulfone ultrafiltration membranes prepared by phase separation method using different macromolecular additives, Journal of Membrane Science, 327 (2009) 125-135; and Weifeng Zhao, Jingyum Huang, Baohang Fang, Shengqiang Nie, Nan Yi, Baihai Su, Haifeng Li, Changsheng Zhao, Modification of polyethersulfone membrane by blending semi-interpenetrating network polymeric nanoparticles, Journal of Membrane Science, 369 (2011) 258-266). In comparison to the number of studies related to organic casting solution additives, the number of studies on inorganic salt casting solution dopants is minimal. Thus, research into novel inorganic dopants may result in new methods to control membrane properties and a better understanding of the phase inversion process.